Broadening the applications of the atom probe technique by ultraviolet femtosecond laser

Laser assisted field evaporation using ultraviolet (UV) wavelength gives rise to better mass resolution and signal-to-noise ratio in atom probe mass spectra of metals, semiconductors and insulators compared to infrared and green lasers. Combined with the site specific specimen preparation techniques using the lift-out and annular Ga ion milling in a focused ion beam machine, a wide variety of materials including insulating oxides can be quantitatively analyzed by the three-dimensional atom probe using UV laser assisted field evaporation. After discussing laser irradiation conditions for optimized atom probe analyses, recent atom probe tomography results on oxides, semiconductor devices and grain boundaries of sintered magnets are presented.     

Mechanism of laser assisted field evaporation from insulating oxides

To explain the recent successful three-dimensional atom probe (3DAP) analyses of insulating oxides by laser assisted field evaporation, we investigated the mechanism of the laser-induced field evaporation of oxides by ab initio calculations. The calculated potential energy surfaces (PESs) for the ground and excited states indicated that the activation barrier height for field evaporation is substantially reduced by the accumulation of holes near the tip apex. This would make the direct electronic excitation possible to promote field evaporation along with thermal excitation. These theoretical calculations are supported by experimental observations.     

Pulsed laser atom probe analysis of semiconductor materials

As the size of semiconductor devices is reduced the active volumes of material in each device is also decreased. Under these circumstances it becomes more important to understand the microchemistry of semiconducting materials, as small fluctuations in composition can dramatically affect both the operation of the devices, and of the contacts to semiconductors. Atom probe microanalysis has been shown to be able to analyse the microchemistry of metallic materials with plane-by-plane resolution, and by using a pulsed laser to replace the more conventional voltage pulses the analysis of semiconducting and insulating materials becomes possible. The pulsed laser atom probe has been shown to give very accurate chemical analysis of the stoichiometry of extremely small volumes of III-V compound semiconductors, and the composition of the interfacial layer between silicon dioxide and silicon has been identified as SiO of thickness about 0.3 nm. It has been shown to be possible to prepare specimens for analysis from thin films of semiconductors, thus allowing the microanalysis of a wide range of materials that are deposited in thin film form.     

Investigation of wüstite (Fe1-xO) by femtosecond laser assisted atom probe tomography

In this paper, we report results obtained from laser assisted three-dimensional (3-D) atom probe tomography (APT) on wüstite (Fe_1−xO). Oxides are generally insulating and hence hard to analyse in conventional electrical assisted APT. To overcome this problem, femtosecond laser pulses are used instead of voltage pulses. Here we discuss some aspects of pulsed laser field evaporation and optimization of parameters to achieve better chemical accuracy.     

Laser assisted field evaporation of oxides in atom probe analysis

We have investigated the laser assisted field evaporation phenomena of ZnO, and MgO to explore the feasibility of quantitative three dimensional atom probe analyses of insulating oxides. To assist the field evaporation of these oxides, the usage of short wavelength 343 nm ultraviolet (UV) laser was found to be more effective than 515 nm green laser. We observed field ion microscopy (FIM) image expansion and mass peak shifting when 343 nm laser was irradiated on MgO. This phenomenon can be attributed to the laser induced electron excitation which causes the reduction of the resistivity of the specimen.     

New tomographic atom probe at University of Muenster, Germany

Currently atom probe tomography provides the highest spatial resolution compared to all other volume analysis techniques. Owing to its single atom sensitivity, it is specially suited to study nano-structured materials. Therefore, a new atom probe was installed at the Institute for Material Physics at University of Muenster, Germany, to study thin film reactions. Since the available budget was rather limited, a cost-effective non-commercial atom probe was constructed. The instrument is based on a 2D delay line detector system of 120 mm diameter. To achieve a large collecting angle and thus large volumes of analysis, a straight flight tube without a reflectron is used. This way, the flight distance may be reduced down to 160 mm. However, the variable chamber layout allows using a reflectron as an alternative. Furthermore, a laser system is implemented that delivers pulses in the 500 ps range to make possible laser-assisted evaporation of atoms. The article describes instrumental details and presents first characteristic data.     

Some aspects of atom probe specimen preparation and analysis of thin film materials

Some of the factors in the preparation of atom probe specimens of metallic multilayer thin films have been investigated. A series of Ti/Nb multilayer films were sputtered deposited on n-doped Si [001] substrates with either 5 or 0.05Omega cm resistivity. Each wafer was pre-fabricated into a series of 5 microm x 5 microm x approximately 80 microm island posts by photolithography and reactive ion etching. Once the film was grown on the wafer, a Si post was mounted to either a tungsten or stainless steel fine tip needle that was mechanically crimped to a Cu tube for handling. The specimen was then loaded into a Focus Ion Beam instrument where a sacrificial Pt cap was in situ deposited onto the surface of the film and subsequently annularly ion milled into the appropriate geometry. The Pt cap was found to be an effective method in reducing Ga ion damage and implantation into the film during milling. The multilayers deposited on the high resistivity Si exhibited uncontrolled field evaporation which lead to high mass tails in the mass spectra, a reduction in the mass resolution, high background noise, propensity for "flash-failure", and a variation in the apparent layer thickness as the experiment elapsed in time. The multilayers deposited on lower resistivity Si did not suffer from these artifacts.     

Some aspects of the field evaporation behaviour of GaSb

In-depth analysis of pulsed laser atom probe tomography (APT) data on the field evaporation of the III-V semiconductor material GaSb reveals strong variations in charge states, relative abundances of different cluster ions, multiplicity of detector events and spatial correlation of evaporation events, as a function of the effective electric field at the specimen surface. These variations are discussed in comparison with the behaviour of two different metallic specimen materials, an Al-6XXX series alloy and pure W, studied under closely related experimental conditions in the same atom probe instrument. It is proposed that the complex behaviour of GaSb originates from a combination of spatially correlated evaporation events and the subsequent field induced dissociation of cluster ions, the latter contributing to inaccuracies in the overall atom probe composition determination for this material.     

On the many advantages of local-electrode atom probes

Local extraction electrodes offer several crucial advantages for operation of atom probes. Because of the proximity of the local extraction electrode to the specimen, the electric field produced at the specimen apex by a given voltage is enhanced and the voltage required for field evaporation is reduced. In a voltage-pulsed atom probe, the absolute magnitude of the energy uncertainty is correspondingly reduced. High mass resolution (m/deltam > 1000) may therefore be obtained by accelerating the evaporated ions to a greater total potential after the local extraction electrode. The low extraction voltage may also be pulsed rapidly (100 ps rise time) and at high repetition rates (up to 10(5) pulses per second) using currently available solid-state pulsers. Furthermore, a local electrode and intermediate electrodes may be used as optical elements to control the image magnification. All of these benefits may be applied to any type of atom probe. Local-electrode atom probes (LEAP) should be especially advantageous for developing three-dimensional atom probes with high mass resolution and a large field of view. A sample has been developed that consists of many microtips formed on a planar sample using ion beam mask etching. Microtip samples are especially suited to LEAP. Analysis of electrically insulating samples may also be possible with microtip samples in a LEAP. This combination of features suggests flexible, high speed, high mass resolution atom probes that can work with either conventional needle-shaped specimens or the new style of planar microtip specimens.     

Pulsed-laser atom probe studies of a precipitation hardened maraging TRIP steel

A precipitation hardened maraging TRIP steel was analyzed using a pulsed laser atom probe. The laser pulse energy was varied from 0.3 to 1.9 nJ to study its effect on the measured chemical compositions and spatial resolution. Compositional analyses using proximity histograms did not show any significant variations in the average matrix and precipitate compositions. The only remarkable change in the atom probe data was a decrease in the ++/+ charge state ratios of the elements. The values of the evaporation field used for the reconstructions exhibit a linear dependence on the laser pulse energy. The adjustment of the evaporation fields used in the reconstructions for different laser pulse energies was based on the correlation of the obtained cluster shapes to the TEM observations. No influence of laser pulse energy on chemical composition of the precipitates and on the chemical sharpness of their interfaces was detected.     

Field evaporation behaviour in the gamma phase in Ti-Al during analysis in the tomographic atom probe

A Ti-48 at% Al alloy has been successfully investigated, using atom probe field ion microscopy and transmission electron microscopy. After a specific heat treatment, this alloy has a (alpha2 + gamma) lamellar microstructure. Using the tomographic atom probe (TAP), it has been possible to image the stacking of superlattice planes of gamma and to identify titanium as the highest evaporation field element. In addition, the influence of analysis site on atom probe measurements has been estimated for this phase. A TAP analysis has also made it possible to observe an extremely thin step along a gamma/gamma interface at a near atomic scale.     

A study of phase separated Ni66Nb17Y17 metallic glass using atom probe tomography

Microstructural characterization of Ni₆₆Nb₁₇Y₁₇ as spun metallic glass ribbon was carried out using atom probe tomography. A comparison of different experimental conditions for pulsed laser and pulsed voltage field evaporation reveal that the laser pulsing can be optimized to avoid preferential evaporation of yttrium. Atom probe tomography measurements illustrate that the sample undergoes phase separation resulting in two interconnected phases during the process of vitrification. The yttrium-enriched phase was depleted in niobium and yttrium-depleted phase was enriched in niobium. Moreover, detailed analyses of the roller-contact and non-contact sides of the melt-spun ribbon show different wavelength of phase separated regions revealing that the degree of phase separation is directly associated with the cooling rate.     

Some aspects of the silicon behaviour under femtosecond pulsed laser field evaporation

Three dimension atom probe analysis of semiconductor materials requires the ability to bring high electric field at the specimen apex to remove atoms. It is shown that, if voltage pulses are used to evaporate doped silicon, the resistivity of the material has to be lower than about 10(2) Omega cm. To overcome this problem, voltage pulses have been replaced by femtosecond laser pulses. The laser pulses give rise to field evaporation by two processes. Both thermal and optical field evaporation have been observed. Thermal evaporation takes place at high laser intensities and with short wavelengths while the evaporation is assisted by the rectification of the optical field for lower intensities and in the infrared domain. Using the optical field evaporation, reproducible and good analyses in term of spatial and mass resolutions could be conducted.     

Quantitative atom probe analyses of rare-earth-doped ceria by femtosecond pulsed laser

We have investigated the irradiation conditions of femtosecond laser pulses for quantitative atom probe analyses of rare-earth (RE) doped ceria. The influence of laser wavelength, power, pulse frequency, as well as specimen temperature on mass resolution and background noise of atom probe mass spectra were investigated. Furthermore, quantitative atom probe analysis of yttrium distribution in Y-doped ceria was carried out with the optimized evaporation conditions. The distribution of yttrium was found to be uniform within the grains, but they were confirmed to be segregated at grain boundaries.     

Effect of laser power and specimen temperature on atom probe analyses of magnesium alloys

The influence of laser power, wave length, and specimen temperature on laser assisted atom probe analyses for Mg alloys was investigated. Higher laser power and lower specimen temperature led to improved mass and spatial resolutions. Background noise and mass resolutions were degraded with lower laser power and higher specimen temperature. By adjusting the conditions for laser assisted atom probe analyses, atom probe results with atomic layer resolutions were obtained from all the Mg alloys so far investigated. Laser assisted atom probe investigations revealed detailed chemical information on Guinier-Preston zones in Mg alloys.     

Overcoming challenges in the study of nitrided microalloyed steels using atom probe

Nitrided steels are widely used in the engineering field due to their superior hardness and other attractive properties. Atom probe tomography (APT) was employed to study two Nb-microalloyed CASTRIP steels with different N contents. A major challenge of using APT to study this group of materials is the presence of tails after Fe peaks in the mass spectra, which overestimates the composition for alloying elements such as Nb and Cu in the steels. One important factor that contributes to the tails is believed to be delayed field evaporation from Fe²⁺. This artefact of the mass spectrum was observed to be the most severe when voltage pulsing was used. The application of laser pulses with energy ranging from 0.2 to 1.2 nJ successfully reduced the tails and lead to better compositional measurement accuracy. Spatial resolution in the z-direction (along the tip direction) was observed to be less affected by changing laser energy but deteriorates in x-y direction with increasing laser energy. This investigation suggests that pulsed-laser atom probe with ∼0.4 nJ laser energy can be used to study this group of materials with improved mass resolution while still maintaining high spatial resolution.     

Time-of-flight atom-probe field ion microscope analysis of thin films: Surface segregation of alloys and early stages of silicide formation

Basic information needed to understand the mechanisms of growth, and the properties and performance of thin solid films includes the atomic structures and compositional variations across the interface layers. This information can be obtained with the atom-probe field ion microscope. The atomic structures of a surface can be directly observed in the field ion microscope. Structures in the bulk can be revealed by gradual field evaporation of surface layers. The composition of each surface layer can be analyzed by pulse field evaporating the surface layer slowly and identifying the field evaporated atoms from their time of flight. Methods of atom-probe thin film analysis are described and results on the composition of thin surface films in alloy segregation and thin silicide films grown on metal surfaces are presented. Four distinctive stages of silicide formation on tungsten emitter surfaces have been observed. This result should be directly applicable to silicide growth on polycrystalline tungsten surfaces.     

Near-field nano-ellipsometer for ultrathin film characterization

We describe a near‐field ellipsometer for accurate characterization of ultrathin dielectric films. Optical tunnelling mimics the absorption in metallic films, enabling accurate measurement of the refractive index of ultrathin dielectric film. A regression model shows that a refractive index resolution of 0.001 for films as thin as 1 nm is possible. A solid‐immersion nano‐ellipsometer that incorporates this near‐field ellipsometric technique with a solid‐immersion lens is constructed to demonstrate the viability of this technique. Such a nano‐ellipsometer can accurately characterize thin films ranging in thickness from subnanometre to micrometres with potential transverse resolution of the order of 100 nm.     

Mirrorlike pulsed laser deposited tungsten thin film

Mirrorlike tungsten thin films on stainless steel substrate deposited via pulsed laser deposition technique in vacuum (10(-5) Torr) is reported, which may find direct application as first mirror in fusion devices. The crystal structure of tungsten film is analyzed using x-ray diffraction pattern, surface morphology of the tungsten films is studied with scanning electron microscope and atomic force microscope. The film composition is identified using energy dispersive x-ray. The specular and diffuse reflectivities with respect to stainless steel substrate of the tungsten films are recorded with FTIR spectra. The thickness and the optical quality of pulsed laser deposition deposited films are tested via interferometric technique. The reflectivity is approaching about that of the bulk for the tungsten film of thickness ～782 nm.     

Atom probe tomography of reactor pressure vessel steels: an analysis of data integrity

In this work, the importance of optimising experimental conditions for the analysis of reactor pressure vessel (RPV) steels using atom probe tomography is explored. The quality of the resultant atom probe data is assessed in terms of detection efficiency, noise levels and mass resolution. It is demonstrated that artefacts can exist even when experimental conditions have been optimised. In particular, it is shown that surface diffusion of some minority species, including P and Si, to major poles prior to field evaporation can be an issue. The effects were most noticeable during laser pulsing.The impact of surface migration on the characterisation of dislocations and grain boundaries is assessed. The importance of selecting appropriate regions of the reconstructed data for subsequent re-analysis is emphasised.